Handle ninja_use_custom_environment_files case in GenerateEnvironmentFiles.

(More fallout from r1612)

TBR=thakis@chromium.org

Review URL: https://codereview.chromium.org/13864006

git-svn-id: http://gyp.googlecode.com/svn/trunk@1616 78cadc50-ecff-11dd-a971-7dbc132099af
Review fix for https://codereview.chromium.org/13864005/, 4 -> len(str)

git-svn-id: http://gyp.googlecode.com/svn/trunk@1617 78cadc50-ecff-11dd-a971-7dbc132099af